Spears's choreographer tells PEOPLE that the pop star plans to join Lauper and others onstage at the Greek Theatre in Los Angeles on June 30 for Lauper's "True Colors" live concert.

O'Donnell says that was news to Lauper (a rep for the tour declined to comment), though Spears has been making semi-surprise appearances as she works on a new album, whose release date still has not been announced.

"If it is happening," says O'Donnell. "I'm thrilled, because I enjoy Britney."

In addition to Lauper, the show is also scheduled to include Erasure, Debbie Harry, The Dresden Dolls, The MisShapes with special guest Jeffree Star and host Margaret Cho. O'Donnell is expected to perform a stand-up routine at the show.

Meanwhile, Spears sent out a new mysterious message herself, posting it on the bottom of her Web site's homepage: It reads, in its entirety, "Mother to Grandmother, and my my, you're grand."